Prioritize forks

[konstin]

Say we have requirements a >= 1 ; sys_platform == 'linux' and a < 2 ; sys_platform == 'darwin'. There's two possible solutions: a==1 and a==2, and only a==1 for both forks.

Currently, it's order dependent:

dependencies = [
  'a<2; sys_platform == "darwin"',
  'a>=1; sys_platform == "linux"',
]

resolves 3 packages (a==1 and a==2),

dependencies = [
  'a>=1; sys_platform == "linux"',
  'a<2; sys_platform == "darwin"',
]

resolves 2 packages (only a==1).

When we resolve for latest versions (the default), we should process the fork a<2 first, since it will resolve to a lower version that the other fork accepts, while a >= 1 resolves to a preference (a==2) that a<2 rejects. When we resolve for lowest versions, it's the inverse. While either way is correct, only a==1 will give us a better (smaller) solution that is consistent between forks.

The same goes for requires-python: A fork with python_version >= "3.10" needs to run before python_version >= "3.12" , since the former allows less version than the latter; here it's independent of the version order.

We need to figure out the exact rules for prioritization considering highest vs. lowest versions, requires-python, lower vs. upper bounds and handling splitting over multiple packages.

[charliermarsh]

Agree with this. I actually think that (if done right) this would've prevented the problematic resolution in #4885. (I don't know if it would solve that problem in general though.)